1. Field of the Invention
This invention relates to neutron logging method and apparatus for investigating an earth formation surrounding a mud-filled borehole and determining characteristics of the earth formation. More specifically, the invention permits a more accurate determination of earth formation characteristics, such as porosity, by compensating neutron logging measurements for borehole mud effects.
2. Description of the Prior Art
Knowledge of the porosity of earth formations surrounding a borehole is important in the petroleum industry to identify possible oil and gas producing regions, to determine whether hydrocarbon gas is present, and to calculate the maximum producible oil index of a formation and other parameters. The porosity of a formation is typically measured by passing a neutron logging tool through the borehole.
Generally, known porosity logging tools have used a high-energy neutron source and two (or more) thermal neutron detectors spaced at different distances from the source. Tools of this sort are described in detail in U.S. Pat. No. 3,483,376, issued to S. Locke on Dec. 9, 1969, and U.S. Pat. No. 3,566,117, issued to M. Tixier on Feb. 23, 1971. In order to determine porosity, a count rate signal from a thermal neutron detector located closer to the neutron source is compared by a ratio circuit to a count rate signal from a thermal neutron detector located farther from the neutron source. This ratio is then converted by a function former or like circuit to a signal representative of formation porosity. Although "ratio porosity" measurements have the advantage that first-order environmental effects on the individual thermal neutron detector signals tend to cancel, residual borehole effects remain which must be corrected by departure curves. These corrections can be positive or negative, depending on the parameter involved and tool design. Certain corrections will be more significant than others. Moreover, the presence of thermal neutron absorbers in the formation and/or borehole complicates interpretation of the ratio porosity signals, and could possibly mask tool response to hydrocarbon gas, especially in shaly formations.
The foregoing and other difficulties encountered with known thermal neutron porosity tools are described at greater length in the copending commonly-owned U.S. patent application Ser. No. 300,418, filed Sept. 9, 1981 by D. Ellis and C. Flaum for Neutron Logging Method and Apparatus for Determining a Formation Characteristic Free of Environmental Effects, now U.S. Pat. No. 4,423,323, issued Dec. 27, 1983 to Ellis et al. Application Ser. No. 300,418 describes improved methods and apparatus for overcoming certain of the residual borehole effects, such as tool standoff and borehole size, in real time so as to provide a more accurate on-line porosity measurement. In the technique there described, two thermal or epithermal neutron detectors are located at different distances from the neutron source. Each detector signal is processed independently from the other detector signal and is separately compensated for the effects of borehole size and tool standoff. The neutron signals are then used with tabulated information to determine formation porosity. Although this "signal processing" technique affords improved results relative to the prior art, it has been determined in accordance with the present invention that still better results can be attained by additionally compensating the detector signals for the effects thereon of the borehole mud.